Reliability is an important business property. Reliability, however, can be difficult to measure in a distributed system comprising many disparate components with differing levels of availability and redundancy. This is particularly true when portions of the service infrastructure are purchased from another company, which may not reveal details of its internal infrastructure. Formal models, end-to-end system descriptions, and simple, uncorrelated modes of failure may be inadequate in more complicated systems in which internal components are obscured from a user.